1. Field
The present invention relates generally to data communication, and more specifically to techniques for reallocating excess power in a multi-channel communication system (e.g., a multiple-input, multiple-output (MIMO) communication system).
2. Background
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to at least an extent, then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A multiple-input, multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. For example, an independent data stream may be transmitted on each of the NS spatial subchannels to increase system throughput.
The spatial subchannels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise ratios (SNRs) for a given amount of transmit power. Consequently, the data rates that may be supported by the spatial subchannels may be different from subchannel to subchannel. Moreover, the channel conditions typically vary with time. As a result, the data rates supported by the spatial subchannels also vary with time.
A key challenge in a coded communication system is the selection of the appropriate data rates, coding and modulation schemes, and transmit powers to be used for data transmission on the available transmission channels based on the channel conditions. The goal of this selection process should be to maximize spectral efficiency while meeting quality objectives, which may be quantified by a particular target frame error rate (FER) and/or some other criteria.
In a typical communication system, there may be an upper limit on the data rate that may be used for any given data stream. For example, a set of discrete data rates may be supported by the system, and the maximum data rate from among these discrete data rates may be considered as the saturation spectral efficiency, ρsat, for any given data stream. In such a system, if each data stream is transmitted on a respective spatial subchannel, then allocating more transmit power than necessary to achieve the target FER at the maximum data rate would result in an ineffective use of the additional transmit power. Even though the excess transmit power may result in a lower FER, this improvement in FER may not be considered substantial since the target FER has already been achieved. The excess transmit power may be more effectively used to increase spectral efficiency on some other spatial subchannels.
There is therefore a need in the art for techniques to allocate/reallocate transmit power among the spatial subchannels in a MIMO system if the saturation spectral efficiency has been reached by at least one of the subchannels.